1. Field of the Invention
The present invention relates to a method for preparing a lithographic printing plate from a presensitized plate for use in making lithographic printing plates (hereinafter referred to as "PS plate") and more specifically to a method for preparing a lithographic printing plates by developing a positive- or negative-working PS plates with an aqueous solution of an alkali metal silicate substantially free of any organic solvent.
2. Prior Art
PS plates are divided into two groups, i.e., positive-working and negative-working PS plates which are in general developed, respectively, with separate developers each specially prepared therefor. For this reason, two kinds of developers must be used when it is needed to simultaneously develop these two kinds of PS plates. In addition, if large amounts of these two kinds of PS plates must be developed, the use of two separate automatic developing machines are required. This is economically unfavorable because of, for instance, expenses and a large area required for the installation of these machines.
Alternatively, if these two kinds of PS plates are processed in a common automatic developing machine, the developer must be exchanged depending on the kinds of these PS plates to be processed. This leads to the reduction of operation efficiency and is likewise economically unfavorable.
To solve such problems, there haves been proposed methods for developing these two kinds of PS plates with a common developer, but they have still been insufficient. This is perhaps due to the following reasons. In general, the developers for positive-working PS plates having a light-sensitive layer, which comprises, for instance, an o-quinonediazide compound and an alkali-soluble resin, are alkaline aqueous solutions mainly comprising silicates and having a pH of not less than 12. However, negative-working PS plates comprising water-insoluble diazo resins and alkali-soluble resins cannot be developed at all or only partially developed with these developers. The light-sensitive layer remaining on the developed PS plates becomes a cause of background contamination during printing. On the other hand, positive-working PS plates likewise insufficiently or only partially developed with a developer for negative-working PS plate and this likewise becomes a cause of background contamination during printing. Moreover, the light-sensitive layer of positive working PS plate is possibly attacked with, for instance, solvents present in a certain kind of developers for negative-working PS plate. In other words, those developer have only limited development tolerancy and the use of such a common developer results in the reduction of ink receptivity and printing durability of the resulting lithographic printing plates.
To solve such problems, many attempts have been made for the development of a method for developing both positive- and negative working PS plates with a common developer, but any satisfactory methods have not yet been proposed. More specifically, when positive working PS plates are developed with a developer composition for negative-working PS plate, the image portions of the positive-working PS plates are partially liquated out and the resulting lithographic printing plates have insufficient printing durability and low ink adhesion, while if negative-working PS plates are developed with a developer composition for positive-working PS plate, the non-image areas thereof cannot completely be removed and this becomes a cause of background contamination during printing.
Under such circumstances, for instance, EP 0399755A and EP 0415422A disclose a method for developing a negative-working PS plate which comprises an alkaline water soluble binder and an alkaline water-soluble diazo resin with a developer for positive-working PS plate.
When the foregoing negative-working PS plate is developed with a conventional developer for positive-working PS plate, a replenisher composition and a developing method such as disclosed in U.S. Pat. No. 4,259,434, the developability of the negative-working PS plate is relatively good in the early stage of the development. However, in the development over a long period of time with the addition of a replenisher, the non-image area is not necessarily be removed completely because of a high ionic strength of the developer and the influence of negative-working light-sensitive resin components liquated out or the like. Further, the non-image area is slightly colored and the resulting lithographic printing plate causes background contamination during printing. Moreover, a part of the light-sensitive resin components which cannot be dissolved in the developer is deposited on the bottom of the developing bath and this becomes a cause of clogging of spray pipes and nozzles.
In addition, Japanese Unexamined Patent Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 64-56442 discloses a method for developing, with a developer for positive-working PS plate, a negative-working PS plate having a light-sensitive layer which comprises an alkali-soluble polymer, a monomer and a photopolymerization initiator. However, the monomer included in this light-sensitive layer is not substantially dissolved in an alkaline developer. As a result, insolubles are accumulated in the developer and an overcoat of, for instance, polyvinyl alcohol for preventing oxygen-blocking is dissolved in the developer as the quantity of processed PS plates increases. This leads to a remarkable increase in the viscosity of the developer and a decrease of the developing capacity thereof.
Further, J.P. KOKAI Nos. Hei 2-220062, Hei 2-219060, Hei 2-217859 and Hei 2-189544 disclose methods for developing a negative-working PS plate comprising a diazo resin and a binder with an alkaline aqueous solution having a pH of not less than 12. However, the development of the PS plates comprising diazo resins and binders with a high pH aqueous solution suffers from various problems such as formation of lithographic printing plates having low printing durability and low ink receptivity due to penetration of a developer into the image area of the printing plates.
Examples of negative-working light-sensitive compositions are those comprising polymers carrying dimethylmaleimido groups as photocrosslinkable polymers as disclosed in U.S. Pat. No. 4,079,041 and those comprising the foregoing polymer and diazo resins as disclosed in U.S. Pat. No. 4,845,009 in addition to the aforementioned compositions.
However, most of PS plates obtained from these light-sensitive compositions are developed with developers containing organic solvents. This is because, if they are developed with developers which comprise a silicate, have a high pH and are free of organic solvents, the developability thereof is greatly lowered and hence the development insufficiently proceeds, while if the developability of the polymers used in PS plates is improved, the sensitivity of the PS plates is lowered and the printing durability of the resulting lithographic printing plate is impaired.